Nuclear Instruments and Methods in Physics Research B3 (1984) 475-478

North-Holland, Amsterdam

475

LONG RANGE TRANSPORT OF ATMOSPHERIC AEROSOL PARTICLES OVER THE MEDITERRANEAN AND ATLANTIC OCEAN

Petra METTERNICH 1.2, H.-W. GEORGII ’ and K.O. GROENEVELD ’

’ Instiiut ftir Meieorologie und Geophysik

’ Institut ftir Kemphysik, Johann Wolfgang Goerhe - Universitiit, D - 6000 Frankfurt/Main, Germany

Particle induced X-ray emission in combination with a size fractionating sampler has been used in the case of long range tranport studies. Chemical and size distribution data and the corresponding air mass trajectories were obtained for aerosols collected over the

open Northeast Atlantic and the Mediterranean. Long range transport of nollutant and natural substances is documented bv the - - characterisation of the aerosol according physical and chemical instrumentation.

1. Introduction

In order to understand and foresee possible effects of global increasing air pollution, the source, transport mechanisms, and sinks of air pollution need to be known in some detail. PIXE is now capable of rapid routine analysis. The properties of this method make it very useful for aerosol studies; large amounts of infor- mation concerning aerosol elemental composition are available. The combination of a size fractionating sam-

pler and a multi-elemental technique has enabled the characterisation of an aerosol and the identification of possible sources and transport processes [l]. We used the PIXE method together with an inertial impactor as a

sampling device in case of long range transport studies of natural and pollutant substances [2]. Elemental par- ticle size distributions of the atmospheric aerosol were determined under the aspect of air mass history at different sampling sites for the ground and the free atmosphere; so far this paper is especially concerned with the Mediterranean and the Atlantic Ocean.

2. Sampling procedure and analysis

Aerosol sampling over the open Northeast Atlantic (40°-60’N) was performed on board the German re- search vessel F.S. “Meteor” during cruise Nr 57 in August 1981 (fig. 1). During October 1981 aerosol sam- ples were collected at Malta, a semi-remote site in the Mediterranean Sea (fig. 1).

The type of collector used in these studies was the single-orifice, seven-stage, Batelle-type impactor of the PIXE International Corp. [3]. In the course of measure- ments at Malta, the sampling device was mounted on a mast, about 2 m above ground level, at the meteorologi-

0168-583X/84/$03.00 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

properties derived from ground measurements and aircraft

cal station at Malta airport. For aircraft instrumenta- tion with a DO28 aircraft an intake nozzle - designed to preserve isokinetic conditions - was mounted below the fuselage, while the sampler and the pumping system were placed inside the cabin of the D028.

On board the research vessel F.S. “Meteor” the sampler was positioned in the forward part of the ship to avoid contamination by the ship exhaust. A wind vane stopped sampling whenever the wind was blowing from astern. No sampling was done when the relative wind was lower than 4 m s-l. Moreover sampling was controlled by a GE condensation nuclei counter, which turned off the sampler when a preset concentration level was exceeded. After each switch-off a 6 min delay was

added to clean the whole sampling system. PIXE analysis was performed at the 2.5 MV Van de

Fig. 1. Expeditions: Northeast Atlantic 1981: route of F.S.

“Meteor”. Malta 1981: ground measurements at Luqa/Malta

airport; hatched area indicates vertical profiles at 3000, 10000,

and 18000 ft.

IV C. AEROSOL APPLICATIONS

416 P. Metternich et al. / Transport of aerosol particles

Graaff accelerator at the Institut fur Kernphysik, Uni- versitat Frankfurt/Main [4].

3. Results and discussion

The samples of the expedition Northeast Atlantic 1981 were grouped according to the history of the corresponding air parcel (fig. 2). For a proper classifica- tion the 48 h backward isobaric trajectories calculated at the 1000 mb level were considered. Due to its geo- graphical position, Malta is influenced by both marine and continental air masses. The meteorological condi- tions included airflow from northwesterly to westerly directions, so the samples were most likely dominated by anthropogenic and terrestrial sources from the European and African continent (fig. 2); some local contributions from Malta may also be present. Con- centration of Aitken nuclei were found between 1000-8000 cm-‘, in good agreement with Aitken nuclei concentrations of about 1000-2000 cmm3 measured by Elliot [5] in the open Mediterranean during strong con- tinental influence from Europe; typical values in purely marine air range from 200-300 cm-‘. The transport of pollutant and natural substances to the Mediterranean is also documented by the results of Arnold et al. [6]: atmospheric concentrations of particulate trace metals are 4-10 times higher than those over the open Atlantic. According to air mass history the relative abundance of heavy metals and soil-derived components changes. During these field experiments at Malta similar ob-

Fig. 2. Air mass origin (schematic): according 48 h backward isobaric trajectories; Northeast Atlantic 1981: calculated at the 1000 mb level, @ purely marine air masses, @ marine air masses with continental influence, @) air masses dominated by anthropogenic sources. Malta 1981: calculated at 850, 700 and 500mb 0.

servations could be made: the elemental concentrations of atmospheric aerosol particles at Malta were about 5-20 times higher than element concentrations mea- sured over the Northeast Atlantic. The marine and anthropogenic components appear to be highly enriched with respect to soil, while the soil-derived elements were found in crustal proportions. The origin of the air mass is also reflected in aerosol chemical parameters plotted against sampling intervals [2]. Significant soil compo- nents represented by Si, Al, and Fe were always present at Malta, showing highly significant correlations with each other. No enrichment with respect to the soil ratios given by Mason [7] could be observed. Ca shows a pronounced enrichment with respect to both K and Cl, without any trend with particle size. Removing the variance of Ca due to Si, the ratio of Ca to K and Cl indicates the absence of soil elements in the fine-particle range. This and the fact that all the soil-derived compo- nents are not appreciably influenced by the actual meteorological parameters makes it unlikely that the Ca enrichment can be explained as a result of the presence of mineral dust but by local sources only. In contrast to the soil-derived components the marine and anthropo- genie compounds of the aerosol are strongly affected by meteorological parameters. Minima of detected con- centrations for all these elements coincide with atmos- pheric processes being responsible for the removal of particles from the atmosphere.

More detailed information about long range trans- port is available from elemental mass-size functions obtained in ground measurements and vertical profiles in the lower and middle troposphere. Fine particle sulphur (< 1 pm ad) shows the most pronounced dif- ference between background and contaminated condi-

SO~/Na+

average

01234567 IMPACTCR STAGE

Fig. 3. Sulphate to sodium ratio (average of all samples) against particle sire at Malta, indicating excess sulphate in both fine

and coarse particle sizes.

P. Metternich et al. / Transport of aerosol pariicles 411

tions; enrichment of elements in very fine particles resulting in a very small mass median aerodynamic diameter (MMAD) can therefore be used to identify aerosols freshly formed by anthropogenic sources. This mode present at Malta, not only in the sulphur mass-size functions, but also observed for excess sulphate with an enrichment of 10-100 fold of that found in the pure marine atmosphere (fig. 3), indicates the strong in- fluence of continental air masses too. MMADs over the Atlantic range between 0.65 and 3.81 pm; small par- ticles dominate in purely marine and anthropogenic air masses, while coarse particles are contributed to ter- restrial sources. For Malta MMADs of about 15 pm were estimated, caused by both terrestrial and anthro- pogenic sources. As reported by Bonsang et al. [8] sulphate aerosol concentrations greater than 10 ng mm3 characterize oceanic areas with high continental in- fluence, such as the Mediterranean; for regions with negligible continental influence concentrations are about 10 times lower. Transport of mineral dust, also as coagulations of small particles, results in large MMADs.

According to these processes MMADs determined for sulphate and the soil-derived components at Malta increase during periods of high pollution (S) and trans- port of desert dust (Si) [9]. In the lower and middle troposphere the excess sulphate cannot be explained by marine sources; it is attributed to continental influence (MMAD -Z 0.25 pm), because both Na and Cl decrease with height. These results agree with observations of Gravenhorst [lo] over the Atlantic: sulphate concentra-

tions remain nearly constant with height, while Cl par- ticles sharply decrease. Cl is enriched with respect to seawater (fig. 4) over the Atlantic in both fine and coarse particles, and at Malta in the fine-particle range only. This and the fact that the MMADs for Cl are always somewhat smaller than those for Na means that the enrichment of Cl in small particles can be attributed to anthropogenic origin; the enrichment in coarse par- ticles is correlated with the production of seasalt par- ticles at the ocean surface by wind.

In conclusion, the characterisation of the atmos- pheric aerosol in the Mediterranean by elemental par- ticle size distributions, air mass trajectories and the comparison with background conditions can be sum- marized as follows: During periods of strong continen- tal influence from the European continent the aerosol of primary marine origin is dominated by pollutant sub- stances from anthropogenic sources. Long range trans- port for most of the anomalously enriched elements may be supported by the variation of the mass-size functions of these elements according to air mass his- tory in the lower and middle troposphere.

This work is supported by Deutsche Forschungs- gemeinschaft/Bonn, Germany through Sonderfor- schungsbereich 73/Atmospheric Trace Substances. The crew of F.S. “Meteor” and DFVLR/Oberpfaffenhofen, Germany is gratefully thanked for their assistance, as well as the Government of Malta for providing the facilities for these measurements.

Cl/Na Northeast Atlinhc ES'Meteor cruise Ne57

AERODYNAMIC CUT-OFF DIAMETER [pm] at Wmin flcwrate -

l . IO' -* _u ,oO..f._.;\ e! ct. . \fl\F . 10-1 _ l l

seawater ratio

J_-__.l.02. 1 I I I I I I I 110-21, , , , , , , I ,()-2[, , , , , , ,I 01234567 01234567 01234567 01234567

IMPACTOR STAGE

Fig. 4. Particle size dependence of CI enrichment with respect to Na for Mediterranean and Northeast Atlantic; sections @ - @ correspond to air mass classification as given in fig. 2; the sets of points in each section represent different samples.

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478 P. Metternich et al. / Transport of aerosol particles

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